Measurement of Molecular Determinants of Body-weight Regulation in the Brain using Magnetic Resonance Spectroscopy.

David Cummings, M.D.

Funded in December, 2004: $100000 for 3 years

What Determines How Body Weight is Regulated in the Brain, and How Malfunctions Result in Obesity?

University of Washington researchers will use a modified imaging technique to explore the molecular processes in the brain that are thought to control body weight and the development of obesity. Using MRS (Magnetic Resonance Spectroscopy) in animals and then in humans, the researchers’ findings could provide insight into key molecular processes and how they malfunction in obesity, which may lead to development of more effective means to intervene.

Animal research has identified a lipid (fat) molecule in rat brain that may play a major role in their weight control, but this has not been translated into human obesity research advances. The investigators’ modified MRS technique may facilitate this translation from animals to humans. Specifically, animal tissues studied in the laboratory suggest that a lipid called FACoA (long-chain fatty acyl-CoA) may regulate appetite and body weight.

First, researchers will measure FACoA molecule levels in the living rat during fasting and compare these to levels seen during feeding, to determine if lipid levels are decreased during fasting. Thereafter, the investigators will use this modified MRS imaging technique in people with diabetes who receive insulin, to characterize the roles of glucose (sugar) and insulin fluctuations in this response. In this way, the researchers will explore the relationship between the lipid molecule (FACoA) and energy-regulating hormones and nutrients, including leptin. These studies should provide information on the role of this lipid molecule in maintaining human energy balance. This study also should provide a refined imaging technique for undertaking further studies in the living animal and in humans of this molecule and others that may play a role in body weight regulation.

Significance: Refinement of MRS imaging to facilitate studies in living animals that can be translated into human imaging research may reveal molecules involved in weight regulation and obesity, which could lead to more effective means to control weight gain and prevent obesity.

Measurement of Molecular Determinants of Body-weight Regulation in the Brain Using Magnetic Resonance Spectroscopy.

Body weight is regulated by a physiological process called energy homeostasis, whereby alterations in body fat stores trigger compensatory changes in appetite and energy expenditure that resist weight loss. In this process, fluctuations in nutrients and adiposity-associated hormones (e.g., leptin and insulin) communicate the status of peripheral energy stores to the brain. The neuronal pathways that mediate the effects of these peripheral "adiposity signals" are rapidly being elucidated in animal models. Information regarding the molecular mediators of energy homeostasis in the human brain, however, is limited because the relevant molecules have not yet been measured in living subjects.

The goal of this proposal is to measure one of these mediators—long-chain fatty acyl-CoA (FACoA) lipids—in the human brain using state-of-the-art, non-invasive, voxel-localized proton magnetic resonance spectroscopy (1H-MRS). Recent data suggest that acute elevations of hypothalamic FACoA (H-FACoA) concentrations exert anorexic effects similar to those of leptin and insulin, and H-FACoA are implicated as possible intracellular mediators of these hormones' actions.

This project will entail the following specific aims:

1. Refine an 1H-MRS technique to measure H-FACoA concentration in rats. We will first optimize MRS acquisition parameters in rats, using the insulin-induced 6-8 fold elevation of H-FACoA content to help identify the H-FACoA spectral signature. MRS data will be validated against the gold-standard measurement of H-FACoA content in tissue samples by negative-ion chemical ionization gas chromatography mass spectrometry (NCI-GC/MS). We will then determine if MRS can detect physiologic changes in H-FACoA induced by fasting, again validating our results with NCI-GC/MS.

2. Utilize 1H-MRSmethods that we have refined and validated in rats to measure the concentration and regulation of H-FACoA in humans. Once our MRS method has been optimized and validated in rats, we will apply this technique to humans. Paralleling our rat studies, initial human experiments will involve pharmacologic insulin administration to facilitate identification of the H-FACoA spectrum. We will then use MRS to measure H-FACoA levels in the fasting vs. fed state, to determine if fasting decreases H-FACoA in humans, as predicted by the model of these molecules as anorexic mediators in energy homeostasis. Based on rat studies using NCI-GC/MS, we anticipate that fasting will decrease H-FACoA content. To examine the individual roles for insulin and glucose fluctuations in this response, we will determine the impact on H-FACoA of fasting during a hyperinsulinemic clamp, under either euglycemic or hyperglycemic conditions. To investigate potential relationships between H-FACoA and energy-regulatory hormones and nutrients, circulating levels of insulin, leptin, ghrelin, glucose, triglycerides, and free-fatty acids will be concomitantly measured in these experiments.

With these studies, we hope not only to shed light on the role of H-FACoA in human energy homeostasis, but also to develop, for the first time, a non-invasive method to measure molecules involved in body-weight regulation in the human brain. In future studies, we plan to apply this technique to quantify other molecules that are hypothesized to participate in energy homeostasis and can potentially be detected by MRS. These include malonyl-CoA, gamma amino butyric acid, glucose, and N-acetylglucosamine. We also hope to expand our future studies to examine brain areas outside the hypothalamus that regulate energy balance.

David Cummings, M.D.

Hypothesis:Body weight is regulated by a physiological process called energy homeostasis, whereby alterations in body-fat stores trigger compensatory changes in appetite and energy expenditure that resist weight loss. In this process, fluctuations in nutrients and adiposity-associated hormones (e.g., leptin and insulin) communicate the status of peripheral energy stores to the brain. The neuronal pathways that mediate the effects of these peripheral "adiposity signals" are rapidly being elucidated in animal models. Information regarding the molecular mediators of energy homeostasis in the human brain, however, is limited because the relevant molecules have not yet been measured in living subjects. We hypothesize that some of thekey brain molecules that regulate appetite and body weight, including long-chain fatty acyl-CoA lipids (FACoA), can be measured non-invasively in humans by proton magnetic resonance spectroscopy (1H-MRS). We propose to use this technique to clarify the roles of FACoA in human energy homeostasis.

Goals:
1. To refine an 1H-MRS technique to measure FACoA concentration in the rat brain, and to clarify its role in body weight regulation.

2. Utilize 1H-MRS methods that we have refined and validated in rats to measure the concentration and regulation of FACoA in the human brain.

Methods:We will first optimize MRS acquisition parameters in rats. Insulin induces a 6-8 fold elevation of FACoA levels in the hypothalamus, helping to identify the FACoA spectral signature. Thus, rats will be given insulin or placebo injections, and spectra will be obtained following these injections. MRS data will be validated against the gold-standard measurement of FACoA content in hypothalamic tissue samples by negative-ion chemical ionization gas chromatography mass spectrometry (NCI-GC/MS). We will then determine if MRS can detect physiologic changes in H-FACoA induced by fasting, again validating our results with NCI-GC/MS.

Once our MRS method has been optimized and validated in rats, we will apply this technique to humans. Paralleling our rat studies, initial human experiments will involve pharmacologic insulin administration to facilitate identification of the FACoA spectrum. We will then use MRS to measure FACoA levels in the fasting vs. fed state, to determine if fasting decreases FACoA in humans, as predicted by the model of these molecules as anorexic mediators in energy homeostasis. To investigate potential relationships between FACoA and energy-regulatory hormones and nutrients, circulating levels of insulin, leptin, ghrelin, glucose, triglycerides, and nonesterified fatty acids will be concomitantly measured in these experiments.